HARQ is a well-known method of error recovery used, for example, in mobile WiMAX, Third Generation Partnership Project (3GPP) UMTS and Long Term Evolution (LTE), and cdma2000 1x-EVDO standards based access networks networks. In HARQ implementations, a receiver may request retransmission of a data packet if an error-free packet was not received. Received soft information corresponding to the defective packet may be stored, however, and used by the receiver in combination with one or more retransmitted packets to recover the originally transmitted data. A HARQ “process” is initiated upon initial transmission of adata packet, and continues to remain in the queue (i.e., awaiting allocation of resources for transmission upon receipt of a negative acknowledgment for a previous transmission) until it until it is terminated. The process may be terminated, for example, when the receiver sends an acknowledgement signal (“ACK”), a time limit is reached, or the number of retransmissions reaches a maximum permissible value.
In typical wireless networks, for example, a base station serves a plurality of users. One or more users are scheduled for service in each of a plurality of scheduling instances. (In certain types of networks, scheduling instances are referred to as “frames,” “Transmission Time Intervals” or “time slots”). Each user may have one or more HARQ processes waiting to be served. Once a user has been selected for service in a given scheduling instance, network resources must be allocated to one or more of the selected user's existing HARQ processes in the queue. In current systems, a new process process may be “instantiated”, i.e., enabled for transmission, if there are no existing processes in the queue or if the existing processes are locked. A process is said to be locked if it is still awaiting an acknowledgment or negative acknowledgment from the receiver in response to a transmission attempt. In the context of wireless networks, the resources to be allocated are air-interface resources. In the specific context of networks that employ OFDMA as a multiplexing technique, such as WiMAX or 3GPP LTE networks, resources are allocated in time-frequency units. Each such unit consists of a number of OFDM symbols and a number of frequency subcarriers (referred to as subchannels in mobile WiMAX).
Previous approaches to the selection of HARQ processes for transmission have applied a first-in-first-out (FIFO) policy, which favors the transmission of the oldest packets ahead of newer packets. Such an approach has generally been preferred because it tends to reduce out-of-sequence delivery, and also reduces the accumulation of large packet-transmission delays.
However, strict adherence to a FIFO policy may under at least some circumstances lead to inefficiencies in the utilization of network resources. Accordingly, there is a need for further process-selection methods that may achieve more efficient resource utilization.